If you look to the right there will be a link list I called "useful websites". There are not many but there will be more with time, I'm sure. These are websites I use very often: for reference, identification or as a shop for plants, seeds and pumice. As for shop websites, I'm aware that there are many good shops out there but I chose these two for the link list, because I've already tried them out. ;)
This website has the best lithops locality data list or LLD (C001 - C416). It's very easy to use and very helpful if you happen to know the Cole-number of a lithops but not the proper name. This way you can look up its looks (roughly) and flower color all in one step. You can change the order settings as well. Of course it also has a gallery. But I wouldn't recommend it for plant identification because the most pictures only show one single grown-up plant at only one particular time during the year, neglecting the variations within a species and changes in color and pattern throughout the year. For proper identification check several websites, not just one. ;)
I think this is the best way to lay your hands on plants and, most importantly, pumice stones if you live in Germany. Uwe Beyer has a large assortment of succulent plants and seeds. Buying plants you can select the age and size, and the prices are very reasonable. The search function is also very helpful. (Paypal is accepted, worldwide shipping)
I've only discovered this shop for me recently and the variety of the offered lithops seeds is amazing. You can get cultivars as well as seeds with Cole-numbers (or just with features you might find appealing), so if you're looking for rare seeds Francois Hoes is probably your man :) You can place an order by writing him an email. (Paypal is accepted, worldwide shipping)
If you stay on Francois Hoes' website you'll find a lithops gallery that I personally prefer for identification: the photographs are realistic, naturally lit and show different plants of the same species side by side illustrating the range of variations quite nicely. For artistic pictures - see below.
This was the first website I looked through and thought "I want this one, and this one, and this one!" I'm not sure whose plants they are, but they are shaped and colored so perfectly it took my breath away. There is also a plant shop there, but I haven't tried it out yet.
You can really meditate browsing in these lithops galleries using the 'slideshow' feature. 木易可儿 is mostly showing beautiful photographs of individual plants, while Nando takes you for a walk through the collection. Also, make sure you're on holidays, because you might get stuck there for quite a while.
This website is in korean so that I can only guess how it works (unfortunately it's full of flash (?) and restrictions most korean websites sadly have). If you scroll down and click on the tag "lithops" you'll get to a gallery with really big beautiful macro photographs. Often one plant is being portrayed during all the phases it goes through in one or more years. It's quite interesting, even though the browsing is inconvenient.
Well, that's all for now. I hope you'll find these "useful websites" useful for you as well.