The lack of 'the' in the title is important. You could us 'in' I guess. This post doesn't look at Pipettes material, but rather stuff featuring members of the band.
Much of this I have not heard but will mention for completion's sake. Co-founder Monster Bobby has a fairly well-received solo album out (Gaps) and there are strong rumours of a second on the way. The other musicians in the band have also contributed to other bands.
But most folks focus on the three vocalists. Becki ('The One With Glasses') contributes vocals to two tracks on Blind Cowboy's 2005 EP (the group also features one of The Cassette - the shadowy background musicians in the Pipettes) and also sings on "When U See The Future" by Y.Misdaq from the album Flowers & Trees.
And now stuff I actually have listened to. Rose ('The Brunette') takes vocals on The Young Plaything's "Life Is Great" from the single Yr So Fit (For Me). This single is available for download on emusic. The song comes from the poppy side of 'Pop 'n' Roll'. Lots of guitar. Rose's vocal isn't the best I've ever heard from her, but still very listenable. Recomended for Pipettes fans and guitar-heavy indie pop fans. The pop being guitar-heavy, not necessarily the fans.
Gwenno ('The One That Hits The High Ethereal Notes') has a MySpace page on which she has posted demos of solo material. The vocals are fairly close to 'final quality'. The instrumentation itself, however, can best be described as Casio-keyboard rough. The main attractions are the vocals and songwriting, so we won't judge the songs on the keyboards (they crack me up, though). The songs are more modern electropop than retro-flavored vocal group. My favorite of the batch is "U & I", which makes much better us of akward phrases than Pipettes' material sometimes does. (It fits 'running around like headless chickens' in!) The songs all have a great sense of melody - very hummable. Again for completion's sake Gwenno's two multilingual pre-Pipettes albums should be mentioned. 2002's Mor Hud and 2004's Vodya.
There. I mentioned them.