Besides many of the excellent choices mentioned so far, I have to plug John Mayer.
He's new, and he has a reputation for being either:
1. A teenybopper
2. A Dave Matthews ripoff
In fact, he's neither. He's actually an incredible guitar player and songwriter. You wouldn't necessarily know this based on the two overproduced singles that have hit the radio and MTV over the past few months. But a live John Mayer concert is something truly amazing.
He has three CDs out now. The first was an EP he recorded which is mostly just him and his acoustic guitar doing eight of his earliest songs, called Inside Wants Out. It's a good introduction to his music and songwriting abilities, though it doesn't showcase his virtuoso guitar playing. The second is a polished, but overproduced album called Room For Squares; it has thirteen songs on it and a full backing band. Well worth picking up if your primary concern is well-done songwriting. Though this one also ignores his amazing guitar skills for the most part.
The third one just came out, and is available either as a double-CD or as a DVD, called Any Given Thursday. It's a complete live show he did in Birmingham, Alabama in September 2002. It showcases all of his talents -- songwriting, guitar playing, live performance, etc. I'd recommend the DVD.