Blood Bowl is such a fantastic set of rules that people have created variant rulesets, and in 2015/16 the NAF Variants Subcommittee got together to produce an Official version for each one. Dungeonbowl, Deathbowl and Sevens tournaments are recorded in the NAF database as Variants in the rankings.
Official NAF Variant Rules
Blood Bowl Sevens
BB7s is a very popular variant, as it can be played in a lunch hour! GW first came up with a set of rules but these have gone from the GW website. The New Zealand forum includes a downloadable pitch.
Streetbowl is played in a confined space with walls and a hard floor, making it more bloody than Blood Bowl! The main NAF rules are above, and there are other versions around such as Street_Bowl_Rules_2012.
The Australian Dungeonbowl has its own website here, with an updated set of rules for CRP, and the other most common occurrence of Dungeonbowl is in the final of the Major of that name in Germany each year. The original Dungeonbowl rules were written by Jervis Johnson.
Dungeon 7s is a Variant that combines Dungeonbowl and Sevens.
Each year in Canada there is a four-way Death Bowl tournament, details of which can be found on the NAF Canada site.
There are other less common variants too!
Not very tested, but there seemed to be a gap in the market, so if you want to try 3-player Blood Bowl try using these rules. Bit of construction required for the pitch though!
Rat Ogre Bowl
One for the Saturday evening of a tournament, often accompanied by liquid refreshment of your choice – everyone uses one rat ogre and tries to score a Touchdown! Rules here.
Similarly, SquigBowl is fun!
Various leagues play various house rules, from changes to spiralling expenses, bounties on particular players, different ways of choosing MVPs. Some leagues go further and change the rules of the game, and one example of this is Narrow-Tier Blood Bowl, a set of rules which aims to reduce the disparity between the top and bottom teams.
BBgridiron.com and Fumbbl have done a great job of collating various house rules, so do check out those links. Gridiron also has them for Deathroller racing!