the best one we have found is also the cheapest! Don't you love it when that happens?! It's the Tomy Light Show, the one shaped like a star, hang on I'll get you the link -
http://www.amazon.co.uk/Tomy-2008-Starlight-Dreamshow/dp/B00020IPDW/ref=sr_1_1?ie=UTF8&qid=1295785894&sr=8-1
Very portable, easy to take away, simple, and she LOVES it - we had a couple more expensive ones initially that both broke very quickly and needed taken back, and also overstimulated her - this is great! If she's still awake when I put her down for a nap, I cuddle her up with her penguin and stick this one, a HUGE smile breaks out, and once it switches itself off - you can set it to 5 or 10 mins - she's out!
HTH
mummy to little Lady T, a girl who knows her own mind