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
Summary Pricing
GTDNext Web app focussing on the GTD methodology. Full GTDNext review Free up to 200 items, $4 per month or $29 per year for unlimited projects, archive & restore
Todoist Multi-platform task and project management. Widely acknowledged as a capable task management system with collaborative features for sharing work. Free, $29 per year for additional active projects and tasks, additional functionality (reminders, calendar sync, backups, templates)
Nirvana Web based with IOS and Android Apps. Centred on the Getting Things Done methodology. Free, $39 per year for unlimited areas and projects
Doit Streamlined task management tool, apps available for Android and IOS Free, $20 per year for frequent syncing, multi-platform, Evernote/email integration
Zendone Slick, Web based GTD implementation with Evernote Integration. Full Zendone review Free, $50 per year for unlimited Evernote and Google calendar integration
Getitdone Web based GTD implementation . IOS/Android capable and can attach files. Google calendar and email integration. Free for 15 days, $39 per year
Vitalist Clean 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
Smartytask Web based GTD implementation. Simple, clear interface and integrated with mobile apps Free for 1 week, $99 per year
Nozbe Cross platform task management - even iWatch! GTD based system. Comprehensive integration capabilities with Evernote, Dropbox, MS Office etc. Collaborative features. Free for 1 months, $96 per year
Actioncomplete Web 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 available Free for 1 months, $84 per year thereafter