I picked up the Panasonic GF2 camera lately because I wanted something small and really lightweight. Sure you can adapt Nikon or Canon lenses, but that took so much away from being 'small and lightweight'. An excellent solution for Sony NEX and Micro Four-Third cameras are to use CCTV or 'C-Mount' lenses. Here's an excellent video test from Vimeo member Dario Viola showing the quality of these inexpensive lenses. These C Mount lenses are normally used in very small security cameras, but they've become a popular choice for many M43 shooters. You'll find many CCTV lenses with fast F/1.4 and F/1.7 apertures for about $30 dollars.
You can't go too wide without seeing the inside of these small lenses, but for the GH2 you might be able to use crop mode to use just the center of the lens (eliminating the barrel ring on the edges). The most popular sizes for the NEX5 and GF1/2 cameras are the 25mm F/1.4 and 35mm F/1.7. Keep in mind that M43 is a 2x crop, so a 25mm is more like a 50mm F/1.4. I'm picking a few of these up to use on the GF2 and will post results soon after receiving. You can find other C Mount lenses below.