HandyNotes_QuestGivers adds questgiver locations and information to the map, so that you can easily pick up all the available quests in a new area. You will need to download HandyNotes separately if you don't have it installed.
To use the addon, open your world map. You should notice little yellow exclamation marks ("!") - that's where the questgivers are located. If you hover your mouse over one of the !'s, you can see what particular quests are offered by the questgiver and at which levels. If the quest's name is displayed in red, it's too high level for you at the moment, green means you can accept that quest, and blue means that the minimum level for the quest is unknown.
Since the point of the addon is to be able to find all the quests offered in a new area, follow-up quests are not listed or displayed. Questgivers are listed regardless of their quests' required levels, since correct information about those isn't always available.
The code is based on HandyNotes_FlightMasters and HandyNotes_Mailboxes code. The quest data was painstakingly extracted from WowHead. If you like this addon, go click a banner their site to show appreciation, and also please ask them to offer their data in an XML format so that I don't have to scrape the HTML for it.
To report a bug or request a new feature, please visit the Google Code site.