Both How I Met Your Mother (HIMYM) and Friends are amongst my all-time favourite TV shows and they share quite a few similarities between one and another. Both series' main theme is about friendship. Both have a main character ending up marrying another main character (Marshall & Lily/Chandler & Monica). They also both have another main character having a long standing crush and eventually have a relationship with another main character (Ted & Robin/Ross & Rachel). Both have a character work as a professor (Ted/Ross) and mostly importantly for this post they both have an awesome opening theme song.
So, why not combine the two shows together. There are quite a few videos made on Youtube having HIMYM's opening played to Friends' theme song. Here are two of the best (although the first video is clearly head and shoulders above the rest) that I found. Enjoy!