I use Vonage and pay about $20 per month. That is the 500 minute plan at $14.99 plus taxes and fees. Their unlimited use plan is $24.99. We've been with Vonage for about three years now and have had no problems.
Some notes:
Vonage has
lots of features, including caller ID, call waiting, voicemail, etc.
If you wish to port your current number to a VoIP provider, that is possible and is commonly done, however, it will take a little longer to transfer that. When I signed up, I just received a new number and I was making calls immediately and receiving calls to my new number shortly thereafter.
You need a decent Internet connection to have good success with VoIP, but it is the upload speed that matters most. I recommend no less than 256kbps. Typically, a standard broadband connection will offer more than this but some 'lite' packages don't. If you have more throughput, all the better.
Something cool that Vonage offers is called
Vonage Access, which allows people to call a number local to them, input your Vonage number, and call you without paying long-distance charges themselves.
If you decide to go with Vonage, you can PM me and I'll be glad to send you a referral. Though not necessary, people who are referred get their first two months of service for free.