The pictures 1mariner7 posted look very much like Anemonia majano (and probably are).

The ones carl posted look like a species of Anemonia, but I don't think they are A. majano.

Either way they are both consider pest anemones, but are no where near as pesty (is that a word?) as Aiptasia.

I personally would leave them in my tank unless they start to get out of control.  The key is to stop them early.  Don't let them take over your tank.

My $0.02

Jake Pehrson wrote: Rest assured these are not Aiptasia.

Which ones? Any clue what they are?

No worries, I don't mind hitchhikers.

I guess I am confused by your excitement. The majano sounds to be worse than an aptacia since it has the same destructive capabilities and tendencies and it has fewer natural predators. To quote the links that you gave:

WetWebMedia, "These pest are about the size and tenacity of Aiptasia, but are in some regards easier and in some regards more difficult to control.  The usual aiptasia predators will show not interest in A. Majano.  The only reliable predators are large angels (Blueface, Majestic, and Emperors) and a few decidedly non-reef-safe butterflies.
The upside is that they are much more leathery than Aiptasia and if you can get to the base, they can be cleanly plucked from the rock."

Reefs.org, "You probably didnt get as lucky as you think, cause it probably is Anemonia majano , a small anemone which most people consider to be a pest. In most cases, this anemone can reproduce quickly and spread thoughout the tank, and like Aiptasia, can sting its neighbors quite badly. It's unlikely that you will find a biological control such as peppermint shrimp for these, however you probably will have better luck using an injection of limewater if you find that they must be controlled in your tank. For further reading, you can check out Terry Siegel's Article on Aquarium Frontiers..."

Rest assured these are not Aiptasia.

Sorry Carl, didn't mean to hijack your thread. 

But, by way of restitution, looking at the Majano info I think the ones in your picture may in fact be Majano too! See the following:

http://www.reefs.org/hhfaq/pages/main_pages/faq_rock1.htm

and

http://www.wetwebmedia.com/anemoniafaqs.htm

They come in brown and green.  I think your friend and I are lucky to get an Aptasia-type friend!!!

A friend in Kansas Emailed me a picture of his system and I believe that I spotted something that appears to be aiptasia. However, it this is aiptasia, it appears to be larger and darker than what I have seen thus far. Can someone positively identify by the pic below please? Thanks.  PS, yes I see the bubble algae too. That's already been addressed.

Could be Majano anemones.  Here's a picture:

Although yours appear to have bulbous tips...