Places for phpBB Modifications:

• http://www.phpbb.com/mods/ - The phpBB website has a database of modifications, all are generally stable and safe and have to be approved for listing.
• http://www.phpbbhacks.com - phpBB Hacks has a fairly extensive list of modifications, some are also available on the phpBB website, but others are not. Might not necessarily be as stable or reliable, but my experience has been good and the authors all have proven to be reliable. They also have graphics and styles available.
• http://www.phpbbmods.de/hacks_list.php - A list of modifications with links available here. The site is German, but most if not all of the mods are primarily in English.

Places for phpBB Styles:

• http://www.phpbb.com/styles/ - The phpBB website has a database of styles as well as a feature that allows you to preview demos of the styles.
• http://www.phpbbstyles.com - phpBB Styles has several phpBB styles to chose from, my only complaint is that the site is little difficult to navigate and find styles for a site centered around phpBB Styles.