As the developer of Phone Amego, I've had the opportunity to use a few VoIP adaptors and SIP phones in order to support them in Phone Amego. Ironically, many advanced phones are not very good at CTI, so I'd like to describe some of my own experience here.
For home use, a VoIP telephone adaptor and cordless phone may be all that's needed. By far the most popular VoIP adapter is the Cisco/Linksys/Sipura series such as the PAP2T or SPA-2102, recently replaced by the SPA112 and SPA122. These have earned high marks for call quality, flexible configuration, low cost, and general reliability. While very capable, these can be a bit technical to configure. In my experience, the newer SPA1xx variants are not as bug free as the older models.
Part of the original team behind the Cisco/Linksys/Sipura adaptors formed a new company (OBiHAI) to build next generation VoIP adaptors. Their first products are the OBi100 and OBi110 Voice Service Bridge and VoIP adaptor. These are remarkable products loaded with powerful features, but at the same time easy to configure using the OBiHAI web portal. For several of the most popular VoIP providers, it only takes a few clicks to enter your VoIP phone number and password and then let the OBiHAI web portal take care of "provisioning" your device. I'd like to highlight two key features of the OBi devices:
(1) They can be used as a kind of distributed PBX to create your own Internet phone system. If you have relatives oversees with high speed Internet, you can talk all you want at no charge by installing one of these at each location. Using the voice service bridge, you can place calls from any OBi device in your circle of trust. A relative in India could connect to your OBi110 in the US to take advantage of your unlimited nationwide long distance plan.
(2) They interface with Google Chat directly so you can place free calls using Google Voice from a normal telephone without any computer involved.
Phone Amego can work with either of these adaptors to provide on-screen caller ID and call logging, but there is no direct support for dialing at this time. If your VoIP service provider has a click-to-dial service, you may be able to dial using that. Alternatively, a USB modem can be used on the analog phone line.
Finally, if you just want a simple plug-and-play solution, consider Vonage. While not the most flexible, Vonage is simple, works well most of the time (your mileage may vary), and pricing is reasonable. One disadvantage is there is no support for CTI beyond a click-to-dial feature on their website. If you want on-screen caller ID, you'll need a separate USB modem or call monitoring unit.