The GrandCentral service gives you a way to ring all your phones (up to six) from a single number. When a caller dials the free number GrandCentral gives you, all your phones ring and it's up to you to decide which one you want to answer. If you're at home, pick it up with your landline, and if you're out, pick it up with your cell.
Other cool features: caller and visual ID, listening to people as they leave voicemails, MP3 ringtones for the callers, call blocking (YES!), call recording, and voicemail storage. The free trial only gives you 100 minutes a month, and it's $14.99 for the premium version. Best yet, you can pick your own area code, so you can establish a presence in various parts of the country.