Hi Elf
We too love singing, and sing a lot around ds (2 years 4 months), who now sings and requests songs too. I love watching/hearing him discover music.
When ds was 1, I bought the CBeebies CD, which has been a hit in the car since. But it drives me crazy after a (short) while, and is more than a bit limited in style, so we've played lots of our own favourites, especially in the car, which had him requesting Sinatra's "Under My Skin" and Vivaldi's "Four Seasons" at 20 months.
Since putting our whole music collection into iTunes over the summer, ds has had his own growing playlists (one upbeat, one chilled-out), which include a whole range of songs. He'll sit on the sofa for half-an-hour just listening to the songs. Current favourites include The Eagles' "Heartache Tonight", The Carpenters' "Mr Postman" (loves this), and The Beatles' "Let It Be". I'd recommend setting up an iTunes playlist; it makes it very easy to isolate and access the songs you think they'll enjoy.
If you want to have a good old sing song together, you might like to try getting hold of the first book in the Voiceworks series. It's a big fat book full of simply arranged, well known folk/pop/spiritual songs, layed out with words, melody, TAB and chords in an easy key. A lot of them are designed for easy peasy harmonising, and there's one arrangement, which includes Michael Finnigan, where the three of you would sing three different well-known songs which all harmonise together. Lots of a capella fun, in an easy format. It's designed as a schools resource, though, so not cheap; about £40, I think.
HTH, and happy singing.