I've mentioned the free tour that I went on in Sofia, Bulgaria several times, so it must be good, right? :) (I'm not getting paid by them to say this--in all honesty, it was a great tour! See other pictures from our time in Sofia here)
On our tour we went to see St. Nedelya Church. It's architectural style is Byzantine and was originally built in the 10th century (you can read all about it on Wikipedia here). The name of the church is also called, "Holy Sunday Church". I don't remember all the history (hence the reason I had to look up Wikipedia to remember the information above!), but I enjoyed looking at it. I thought it was quite beautiful.