my favorite time tracker is www.myhours.com. I use it for invoices as well. You can export data for invlices but you can also email the time, taasks and charges directly to your clients as needed. and its free.
I have used myhours.com for the past year, but it is very unstable. I finally switched to www.paymo.biz. I like that is has a desktop widget that syncs to their website that allows you to track time without having to login to the website each time. You can keep track through the website also if you are working from different locations. So far I haven't had an issue with stability. It also allows you to send invoices although I don't use this feature because I use QuickBooks. www.paymo.biz is free for up to 3 users, but they also have paid options if you need to add more.
I've joined Paymo as well and am going start using it soon. If you use Client Spot they also have a feature that lets you input the amount of time spent on specific tasks and you can download that into an Excel spreadsheet. It doesn't track the time for you though.