While I was at the Craiglist Nonprofit Bootcamp in Berkeley this weekend, a woman asked me a similar question about her small nonprofit. I just sent her an email highlighting these resources from various places around TechSoup. Depending upon your needs, volume of donations, whether they come in online or through more traditional check/credit card terminal methods, and how you're planning to use and manage the info after you're gotten a donation, can all make a difference in deciding what might fit best.
I've personally used Salesforce, GiftWorks, and eTapestry before, as well as online donation processing services like PayPal and DemocracyInAction.org toolsets. All have advantages and disadvantages (for example, Salesforce is free to qualifying nonprofits and has a tons of bells and whistles, but it can be challenging to set up and maintain since it takes a lot of up-front work to customize because of all the bells and whistles!).
I would recommend spending some time looking at the resources below and then searching within these forums (by clicking "Advanced Search" on the top right of each page) for each of the products you're considering to see what people's individual experiences have been.
General Donor Management Tool Info:
Online Donations/Credit Card Processing Info:
Hope it helps!
Becky