We talk a lot about organising your to-do lists here at Lifehacker, but sometimes it can be just as important to keep track of what you've done. Free service iDoneThis motivates you by keeping a calendar of the tasks you actually accomplish.
iDoneThis is simple: Every evening, it'll send you an email asking you what you did today. You reply, listing all the tasks you've accomplished, and it will put them on a simple calendar. Not only does this help you keep yourself accountable to actually get things done, but scanning through what you got done over the last week or month is great for a little motivation boost. Hit the link to check it out.