xxxHOLiC, Sayonara Zetsubou Sensei Recommendations
There are very similar to each other, although xxxHolic has a much darker perspective than Sayonara Zetsubuo Sensei, and Sayonara Zetsubuo Sensei has a much funnier comedic perspective?
Weird atmosphere, funny characters, some mystery...
It is pretty hard to see the connection between both series, but the style is very similar and the characters (Watanuki and Zetsubou sensei) are similar too. If you liked one, you'll sure like the other.
Similar art style, similar style of humor (although SZS is much darker than xxxHOLiC)...oh, and Nozomu is pretty much who Watanuki would be if he didn't encounter Yuuko.
Both have unique art styles ( a bit similar, although the XXXHolic characters are leaner), characters, and great openings and endings. Sayonara Zetsubou sensei is more comedy oriented, though. Both have dark-ish aspects, but are backed up by comic relief.