Long ago, Mark Waid was trying to wrap up Action Comics Weekly, an ensemble comic that had several long running stories of various characters in it. Neil Gaiman was just breaking into the game, and Mr. Waid thought he might like to have a taste of regular fanboy writing. So he assigned Neil to do a final issue, with all the main characters of ACW's storylines appearing.
No problem! Neil was also interested in fixing some problems with Green Lantern (Hal Jordan) continuity that bugged him, namely an instance where Superman had blown Hal off. Sadly, this was just after Infinite Crisis, and continuity was shifting back and forth. The Superman office said that the events could no longer be in continuity so the issue couldn't be printed. Mr. Waid apologizes to Neil. Neil moves on to bigger pastures.
Time goes by. Circumstances change, and they find that they can now print the book. Alas, but Mr. Waid can no longer find his copy of the script. He thinks of the image described below and cries. Yes. Eventually, Neil finds a copy that had been mailed to a friend of a friend. And that is how we got Superman/Green Lantern: Legend of the Green Flame.
The book was printed in graphic novella form, drawn by various artists. While there are cameos by Blackhawks, and Deadman, the bulk of the story is a buddy get-together with Superman and Hal Jordan, with a deus ex machina by the Phantom Stranger. Continuity-wise, it occurs just before the restart of GL and before Hal gets his grey streaks.
Hal seeks Clark out at the Daily Planet. Because Clark needs to write up a museum exhibit they can really hang out, Hal tags along. Whilst there, Hal finds a lantern remarkably similar to the then-missing Alan Scott-Green Lantern (he was in Odin's pocket dimension fighting Ragnarok). Hal decides--what the hey--he's gonna charge up his ring on that bad boy. Digression: I thought this was a bit stupid of Hal at the time. However, after reading the description of Hal's spiritual view of the whole ring/lantern schtick, I can see that it's like expecting a Christian to not pray on Sunday.
Unfortunately, Alan's old lantern is magic-based, and mixing with Hal's science based ring causes a mystic boom with unexpected results. THIS is what happens next.