On the other hand, this isn't to say though that crossbreds should not be carefully thought out and planned; they should be just as carefully thought out as any purebred, perhaps even moreso, because some crosses just do not yield successful results. Also, this is not to advocate for the breeding of every Heinz 57 - by any stretch of the imagination. I strongly believe that you need to do your research when breeding. This includes knowing your horse's background (having a pedigree) - health genetics, conformation faults, etc, as well as examining current crosses on the market, so that you can determine the best cross for your horse. In a world with so many surplus horses (and now, a dismal economy), we owe it to our horses to breed carefully. Breed the best to the best and make sure there's a niche in the market for the type of cross you're breeding (unless you're breeding for your own uses and anticipate on keeping said foal for life).
So, let's take a quick look at just a few of the successful crosses out there (some of my personal favs):
Clydesdale x Thoroughbred - this usually makes for a good sporthorse cross when you cross the athleticism of the TB with the substance and cool-as-a-cucumber brain of the Draft.
Friesian x Thoroughbred - another great sporthorse cross often particularly suited to dressage. Again the focus is on combining the best traits of both breeds - the athleticism of the TB and the brain and movement of the Friesian.
Holsteiner x Westphalen - of course you've got your warmblood x warmblood crosses as we refine and adjust our warmbloods to create better sporthorses to better suit our specific and current needs.
Morgan x Arabian - these guys make great crosses by toning down the energy of the Arabian, combining the good brain of both breeds, and adding substance to an Arabian/adding more athleticism and a lighter build to a Morgan.
QH x Arabian - another fantastic cross whereby the build of the QH is refined and energy is perhaps even added to the equation, while still maintaining the QH brain and keeping the horse cowy (when bred to cowy QH's).
Warmblood x Thoroughbred - another very popular sporthorse cross, though I hear a rumour Standardbred x Warmblood crosses (when done carefully) can make even better crosses that are perhaps less likely to break down. With this cross you lighten the Warmblood build and perhaps create a quicker, cattier horse with the quieter mind of a WB.
As with any breeding - crosses between breeds need to be done with much deliberation and research.