I recently had to implement spoiler tags in to Co-Opp and had a quick perusal of the nets and found no such phpbb3 plugin/mod. So I figured I'd make one using their good old fashioned custom bbcode page and inline javascript.

In case anyone else needs some inline spoiler tags for phpbb3, or anything else really, do as follows:

Login in to your phpbb3 ACP and click on the Posting tab, then click on the custom bbcode link on the left.

For the BBCode usage, enter:

[spoiler]{TEXT}[/spoiler]

For the HTML replacement, enter:

{TEXT}

Example of what this will look like:
OH MY GOD SPOILERS