Should you stray beyond your trust notebook and pen, here are a few to-do list and productivity apps that are worth a look for task management.

App
SummaryPricing
GTDNextWeb app focusing on the GTD methodology. Full GTDNext reviewFree up to 200 items, $4 per month or $29 per year for unlimited projects, archive & restore
To RoundTo Round is a fun little app that provides simple task lists via an innovative, even novelty interface. While it lacks many of the more advanced features and platform support of the established players, it is likely sufficient for many people’s needs. And, given the excellent price point of $0, it is arguably great value for money! To Round ReviewFree
TodoistMulti-platform task and project management. Widely acknowledged as a capable task management system with collaborative features for sharing work. Todoist for Windows ReviewFree, $29 per year for additional active projects and tasks, additional functionality (reminders, calendar sync, backups, templates)
Moo.doIf you are an avid Google services user then Moo.do offers a novel pane based solution for tying Gmail, Todo List and Calendar together. Moo.do ReviewFree and premium at $7/month or $69/year
SwipesMulti-platform, IOS, Android, Web, Mac. Elegant and simple user interface similar to Todoist. Very simple and quick "Swipe", task management. Votes one of the top 5 productivity apps. Full review of Swipes for Android.Free as of September 2016. Plans to introduce a fee based model at $5 per month.
Omnifocus 2Gets on with the business of managing your tasks in a competent, understated way with a minimum of fuss. Full Omnifocus 2 reviewOmnifocus 2 for IOS devices is $39.99 for standard and $59.98 for Pro.
DoitStreamlined task management tool, apps available for Android and IOSFree, $20 per year for frequent syncing, multi-platform, Evernote/email integration
ZendoneSlick, Web based GTD implementation with Evernote Integration. Full Zendone reviewFree, $50 per year for unlimited Evernote and Google calendar integration
GetitdoneWeb based GTD implementation . IOS/Android capable and can attach files. Google calendar and email integration.Free for 15 days, $39 per year
VitalistClean cut GTD implemetation. Mobile possible as is task sharing and other online tools.Free, $5/$10 per month for basic and premium paid versions offering sharing, file attachments and additional projects
TickTickAn elegant way to manage one’s tasks on the go, while still retaining a comprehensive feature set. Full TickTick ReviewFree, $1.99 per months for custom smartlists, calendar view & task activity.
NozbeCross platform task management - even iWatch! GTD based system. Comprehensive integration capabilities with Evernote, Dropbox, MS Office etc. Collaborative features. Full Nozbe ReviewFree for 1 months, $96 per year
ActioncompleteWeb based Task management leveraging aspects of GTD. IOS and Addroid apps also available. Normal GTD functionality - projects, next actions, waiting etc. Migration to and from capable with csv import and export. Drag and drop task functionality.Free for 1 week, $19.99 per year thereafter
Facile Things"Ultimate solution for true GTDers", need I say more? Good "Stuff", collection possibilities with Evernote, Google alendar, email etc. IOS and Android apps availableFree for 1 months, $84 per year thereafter
TrelloKanban style storyboard of tasks with To do, Doing, Done etc. Visually impressive with collaborative and activity listing functionsFree, $8.33 per month for larger file attachments, additional integration options with web apps, improved organisation of projects
HiTaskSimple, clear interface and the ability to share task lists across team members, File sharing, integration with Google calendar and email and reporting functionality. Full review of HiTask.Free, $192 per year for 5 licenses. Sliding scale for larger teams.
ProducteevWeb based project management for teams. Task chat, file attachment, activity notifications and mobile apps. MS outlook integration possible. Full review of producteevFree, $99 per month for enterprise bringing MS integration and support.
FlowKanban style task management with team chat. Mobile functionality across IOS and Android platforms. Free for 15 days, $19 per month for up to 3 people. Sliding scale for larger teams
DroptaskVisually impressive and visually oriented task management web app where traditional lists are accompanied by circles denoting projects and smaller interior circles the associated tasks. Comprehensive integration with other web apps, gmail, Dropbox, Evernote etc. Multi-platform and even offline working. To get many of the good functions though, payment seems to be a must.Droptask review.Free, $250 per year for 5 users with unlimited attachments/subtasks, improved task functionality (dependencies, emailing, reminders etc) integration with other online apps).
AsanaCollaborative, web based task management. Project capabilities, task based chat, dashboards, file attachments, mobile integration.Free, $8.33 per user per month for unlimited tasls/projects, data export, improved administration
WorkfrontFarily comprehensive web based system spanning project management with Gentt functionality, resource and capacity management, resource levelling, even burndown charts, document management and task based communicationPricing on request, $360 per year for work licenses, $720 per year for team leaders for plan licensesPricing on request, $360 per year for work licenses, $720 per year for team leaders for plan licenses
PodioKanban style team task management. Integration with many online apps (Evernote, Google calendar, Drobbox etc.) file sharing and mobile integration$9 for basic, $14 for plus and $24 for premium per employee per month. Upgrading between the plans enables unlimited workflows and visual reporting.
ZohoMore focused on project management than many other web apps here. Gentt functionality, multi-level subtasks, time sheets, document control for projects, reporting, task notifications and integration with Google appsFree for 1 project, $20 per months for 20 projects, $40 for 50 projects
Priority Matrix for WindowsNovel interface based on the Eisenhow Matrix but undermined by software bugs. Review of Priority Matrix.$24 per user per month.
NirvanaWeb based with IOS and Android Apps. Centred on the Getting Things Done methodology.Free, $39 per year for unlimited areas and projects
SmartytaskWeb based GTD implementation. Simple, clear interface and integrated with mobile appsFree for 1 week, $99 per year