Nubes,
Up until this past winter I had run nothing but nanos (2) 10g's, (1) 20g H, & (1) 20g L. If you have not already visited it, you might want to visit
http://www.nano-reef.com/, and read some of the forums there. It is a site dedicated to reefs that are less then 30 gallons and it can provide you with a TON of information on nano-reefs and what you can do with them.
The type of corals you pick for them is very important! In such a small environment there can be some pretty serious chemical warfare going on between different species. Be VERY careful about adding any species that has long feeders (frogspawn, galaxia, torch's, etc) and DEFINATELY try to research everything before you buy it (don't just go with what the guy at the local fish store tells you).
Additionally, if you intend to keep an anenome, be aware that when they start to move they can do some pretty serious damage to your reef system. I added a small anenome that is non-photosenthetic (it was labeled as a Waratah - though the only anenome I know of by that name comes from tidal zones in Aust.) to a 10g with just some Zoo's, 'shrooms, and xenia and it did great in there. Later on, when it was moved into my 90 gallon, it "went for a walk" through a number of my SPS corals killing some fairly large sections of them. As with any living thing that can move about on its own, you should be aware that they can do this. (He has since settled down, but if I could get him isolated I would still remove it & throw it back into a tank by itself).
Now, havign said all of that, here is a list of things that I have kept in a nano tank:
Mushrooms / Ricordia (assorted types) *
Leathers (assorted types) *
Zoanthids / Palythoa (assorted types) *
Green Star Polyps *
Open Brains *
Montipora (Digitata, Capricornis, Spongodes)
Pocillapora verrucosa
Stylophora pistillata
Candy Cane *
Frogspawn *
Xenia *
I have star'd the items that are great for beginers (mainly softies / LPS). These are low maintenance corals & that can take a bit of a "swing" in water chemestry without being to drasticly effected (as an added bonus they don't require as much light. This means your tank will be easier to keep cool, evaporation will be less, and you can save a bit on lighting.)
You should be aware that in the summer time your tank may require daily fresh water top offs, and you should be VERY diligent about your water changes (particularly if you don't run a skimmer). If you want to avoid having to add water changes all of the time, look into getting an automatic top-off system.
If you want some more information, check out Achu's thread here, I replied to it a while ago, but forgot about it until I got to this point in my response. LOL:
http://www.reeffrontiers.com/forums/showthread.php?s=&threadid=1144&highlight=topoff
If you want to see what my old 20L nano-reef looked like, take a look at:
http://www.reeffrontiers.com/photos_members/showphoto.php?photo=207&password=&sort=1&cat=all&page=1
Well thats my $0.02 on all of that.
~V