many a times, a nice song in the opening credit sets the mood of the audience from the start, not sure how much viewers' attention it grabs for the opening credits
at the same time, it saves the audience from one less forced song&dance sequence out of nowhere in the movie
and gives the song & company(music director, singers, choreographer, dancers etc) a chance to show what they got
E.g. Parineeta - http://www.youtube.com/watch?v=bLUO7O20
at times cartoon sketches are used, many a times related to movie in a way that portrays the theme/plot
Here is one example where the comic characters in the title sequences are directly related to movie - NOTE: cartoon sketch of the actors themselves, not just using the original spiderman cartoon
Spider-man-2 - http://www.youtube.com/watch?v=iSDU2tu7
Se7en has some interesting title sequence with the right of music
http://www.youtube.com/watch?v=yALjuJcf
EVER came across an opening credit where they speak out the credits? (in theatre they do as its feasible)
Bawarchi - http://www.youtube.com/watch?v=lO7DHvt0
title sequence for the sake of it
Enter the void - http://www.youtube.com/watch?v=tPxgi-Pi
flashes the text fast enough for anybody to make anything out of it
Inside man is the first movie I know of which uses a popular bollywood song for its opening credits
http://www.youtube.com/watch?v=wy81dVuS
leave the lyrics, the song's music itself sets the tempo announcing with a BANG that here we are, get ready
contemplative
angry