It started with a lunchtime brainstorm by the guys who make Monday Night Combat: Maybe Valve would like to do a Team Fortress crossover? That began a process that required all of a 15-minute drive and a handshake.

No money changing hands, no strings attached.

"For us, it was like, ‘Really? That's it?'" said Chandana Ekanayake, the art director for Uber Entertainment, a Seattle-area developer located just up the road from Valve. "It was a handshake agreement, completely free."

Team Fortress 2's signature hats, plus Penny Arcade's - ahem - Fruit Fucker will appear in Monday Night Combat for those who order the game before Tuesday. It's the latest in a string of recent high-profile crossovers touching the indie community, with Valve as a player in nearly all of them.

Telltale Games produced "Poker Night at the Inventory," uniting Penny Arcade, Homestar Runner and Team Fortress 2 with its Sam & Max franchise. Super Meat Boy's been extremely visible of late, bringing in a whopping 18 characters from other games across its PC and Xbox Live versions. The headcrab from Half-Life makes an appearance in the version available over Steam.

Robin Walker, the creator of Team Fortress, said Steam availability isn't so much a business requirement for the crossover as it is a design component serving them. "It's hard for us to do a tight connection between two games if they aren't operating within a system where they could ‘talk' to each other," Walker said, "which is what Steam is doing in the crossovers so far."

Certainly, adding something to a game that sells over a service Valve maintains benefits both parties, without the need for additional lawyers or fees paid. But the manner in which this is done creates a sense of indie development solidarity, and gamers have demonstrated their willingness to join that cause.

Valve makes a lot of money with several major brands, is a big player in games development and, through Steam, distribution. It's still an indie company in both philosophy and design. "Their teams are tiny," Ekanayake said. "On the Steam side of things, we dealt with just three people. It is very much indie in that sense. They respect the team, which is really cool."

Once Valve agrees to the use, their symbols and characters are in the hands of another developer. But the discussions about Team Fortress 2 involved that team's members, Ekanayake said, basically Walker and a few others. No brass hats or high-level meetings, just folks who could relate to one another as games creators.

"Our core assumption is that developers of another game understand their game and its community better than we do," Walker said. "The challenge in crossovers is to find a way to benefit the audiences of both games, and legal paperwork just isn't an interesting part of that. It's also hard enough already without placing some arbitrary constraints over what a partner is or isn't allowed to work with.

"Instead, we prefer to start with a wide space of possibilities, and narrow down to good choices through an ongoing conversation, trusting each of us to protect the other from making a decision that's bad for their game or audience," Walker said.

There's a reciprocity; those who have both Monday Night Combat and Team Fortress 2 will see items from MNC's Pro Gear System. So as Uber was figuring out how Valve's property best fit in with its game, Valve was doing the same with Uber's content.

Ekanayake said early plans called for Scout in Team Fortress 2 to get the oversized grinning head of Bullseye, the Monday Night Combat mascot, as a hat. It turns out the item was just too big and unwieldy to be fun in the game, so it was discarded in favor of the rest of the rest of the mascot costume plus a couple of other items.

The crossovers aren't entirely an altruistic thing; the limited availability is meant to drive sales of Monday Night Combat on Steam, which benefits both Valve and Uber Entertainment. Perhaps that's why these content-swapping deals can be done with a minimum of hassle.

Walker said Valve's door is always open. "Different products have different goals and requirements, so what works in TF2 might be a terrible idea with Half-Life 2. But if another developer wanted to do something interesting with our [intellectual property] in their games, we'd be happy to see if it made sense."

In the end, Walker said, a big reason crossovers come to pass is because both sides just think it'd be cool.

"It should be simply about finding more ways to make our customers happy, but I'd be lying if that was the only reason," Walker said. "We're gamers and fanboys too. Sometimes we like to do something fun with the people behind games that we like, especially if they're made by people who worked on games that made us want to work in the industry in the first place."