During the ECW invasion storyline in 1997, Jerry Lawler appeared on ECW programming despite being contracted by the WWE. It was actually a work, as Vince had actually been attempting to help ECW be recognized, with Paul Heyman making several appearances on Raw during the angle.
After ECW World Heavyweight ChampionMike Awesome appeared on the April 10, 2000WCW Monday Nitro while still holding the title, Paul Heyman filed an injunction to prevent Awesome from appearing on WCW TV. This led to the one-of-a-kind situation where Tazz, a WWE-contracted wrestler, defeated Awesome, a WCW-contracted wrestler, for the ECW World Heavyweight Title.note Tazz then lost the belt to ECW's Tommy Dreamer.
Perhaps the most well known crossover in wrestling happened on the last episode of WCW Monday Nitro, March 26, 2001. After Vince McMahon's purchase of the promotion, a simulcast was scheduled, with Vince on Raw bragging about how he bought his competition, while Nitro featured the WCW talent wrestling for perhaps their last time on television. The final minutes, however, proved to be the most memorable. On Nitro, Shane McMahon made an appearance on Nitro (in Panama City, FL), while Vince was at the Raw location (in Cleveland, OH). The titantron showed Shane on Raw, while Nitro had its Nitrovision show Vince. The segment was to begin the invasion storyline (and that, in kayfabe, that Shane was the actual owner). It was memorable in that both TNT and TNN were basically broadcasting the same thing during those final ten minutes or so, but in different settings.
Anything that can be deemed an "interpromotional match" can be considered a crossover of one talent for another promotion.
After the WWE brand split, any match that had a Raw talent going up against a SmackDown! talent was considered this, and thus, you got a good bit of cross overs between the two brands. This was further jettisoned with the "Supershows", where Raw and Smack Down stars were on the same shows (to the point where from August of 2011 to the Raw 1000th episode, Raw was renamed the "Raw Supershow" with the word "supershow" being tacked on to the Raw logo in the style of the Smack Down logo.
TNAused to do crossovers with Ring of Honor and CMLL. They persevered longer doing crossovers with AAA, "team TNA" being a regular fixture there in triple's matches.
Ring Of Honor had a quite a few, its Age Of The Fall stable appearing in Full Impact Pro, it housing "Shimmer matches" including the title defenses. Its "feud" with Combat Zone Wrestling. Featuring stars from Japanese promotion NOAH. It has also sent its champions out to defend their belt on the shows of other promotions such as TWC in the UK.
Besides Ring of Honor, CZW used to regularly crossover with Big Japan Pro Wrestling. That was, until a CZW booker walked out of BJPW with one of their titles. CZW has also been invaded by IWA Mid-South and Maven Bentley Productions (the latter seems kind of redundant, as they have almost all the same talent on their rosters anyway).
CMLL frequently has them with New Japan Pro Wrestling. Usually benign as sharing of talent can be but a group from NJPW called La Ola Amarilla are a regular thorn in CMLL's side.
Kaiju Big Battel used to have ongoing story lines with Chikara Pro. Two of Chikara's biggest angles, The Order Of The Neo Solar Temple and The Bruderschaft des Kreuzes only being possible because of this interaction despite most of the wrestlers in both groups have little direct contact with Big Battel itself.