One of my favorite games of all time has to be Ultima Online, I grew up with it as my first computer game I played. I feel very nostalgic whenever I hear the music from that game, and it's stuck in my mind for many years.
So, it inspired a project of mine, to make portals that draw inspiration from the moongates of UO. I started with blue and red, then discovered the other colors moongates have appeared as and just rolled with it and made many variants. I made it all while listening to the soundtrack of UO, it was a pretty enjoyable experience, for having messed with emitters the whole time, which are really obnoxious in how they behave.
Also, I realize there are already moongates (or so it appears) that have been made, although they are more old school than the design I went for here. Also, these are based off of the design, and not exact designs. I never played the earlier installments of the Ultima series, so I only felt nostalgic about the oval-shaped moongates. The moongates are selectable placeables, and I will be posting a download link soon-ish*.
Here are some pictures:
For more images, click here: http://imgur.com/a/8KdKv
*Soon-ish as defined by being somewhere in-between Soon & End of Time.