Depends on what your needs are. I've used a few different ones over the course of my career. Initially I used ACT by Sage. I liked the fact that I was able to fully customize fields, tabs, etc. (if you are tech savvy then this may work for you). You can do mail merges to print letters, handle your email, etc.
I then switched to eBrokerhouse. I'm an reo agent and while act was sufficient to keep track of everything, I liked that it was online.... So the 7 agents/assistants working under me could also have access to any of our property info. It also has a section for inputting your buyer contacts, offer submission and management and more.
In the past several months I've integrated sendoutcards...
Import contacts, create campaigns, completely customize cards and include personal photos of my customers when they close a house for example (and optionally include gifts or gift cards). You create it, but they print it, stuff it, stamp it and mail it all for you. I needed something like that for a long time as my busy schedule didn't leave much time to handwrite notes, drive to store to buy gifts and post office runs, etc. You know the drill... Not enough time in the day. If you want to check it out with a free trial it's
www.thatcardsite.comI've also experimented with agent office, top producer and the CRM tools provided by my current and past brokers but I found my options were limited... So again depends on what features your looking for.
Hope this helps and good luck.