Do you want to start a currency in your community, or just see what a transction in another form of currency might feel like? There are lots of free tools available that can help you do that. What follows are the ones that I think are the most interesting:
1) The Flowplace. The flowplace comes out of The Transitioner, based in france, and in their own words, "...allows individuals, communities and organizations to build wealth together by using free currencies." For me what makes the flowplace especially interesting is that it is a demonstration of the Open Rules concept that the MetaCurrency group has been pioneering. For the sake of disclosure, I am perhaps a little biased, as I have been deeply involved with MetaCurrnecy for the past year or so. While the flowplace doesn't have the most polished interface yet, it is a powerful demonstration.
2) WikiWikiMoney. If you are an avid twitter user, then WikiWikiMoney may be for you. It allows twitter users to set up currencies in a few simple steps. Users even have the option of making their currnency mutual credit. To make a transaction simply tweet the payment. WikiWikiMoney will find your tweet in the public timeline, and update accounts accordingly. Unfortunately, there are limited currency design options, and all transactions are public. However, this might work great for a timebank where it doesn't really matter if transactions are private.
3) Matthew Slater's mutual credit drupal module. For those of you who don't know about drupal, it's great. THE MONEY FIX site was made using drupal, and so far I have had a great experience. Drupal modules are bundles of functionality that you can plug into your website (as long as it is built with drupal). Matthew Slater build this module to run mutual credit currency systems off of drupal sites. The nice thing is that it is free and open source. If you want to set up your site with drupal, this gives you a pretty nifty way to do currencies.
4) Cyclos. Cyclos probably offers the most functionality of any open source currency software. There are quite a few groups using cyclos around the world. I have heard that it can be a little unwieldy, but I have not actually used it myself. It may also take a little more tech savvy to properly install an instance of cyclos than the other platforms mentioned on this list. However, if you have the know-how, cyclos can most certainly be a powerful tool.