In addition, to the list Eve provided I would also include items such as emails, meeting minutes should be retained in some repository.

I would also want a tool that could show me the relationships between the various information source such as Microsoft Outlook’s journal function to show a time line of creation and a simple version history. Maybe a tool such as subversion or JIRA to retain the versions, decisions and issues for Knowledge retention.

An application I have looked at using is Personal Brain http://www.thebrain.com/ to contain all of this information and represent the relationship between the data/information.

Having said this, here are a few responses to your post. Mine are decidedly from the IT side.
– updated specs (many times, specs are not kept current)
– mapping documents
– standard templates (traceability matrix, use cases, etc.)
– chart and/or documentation of existing applications (including one’s own, if any of it has already been implemented)
– access to appropriate software to do job (Visio, Photoshop for mockups, etc.)
– glossary of business analysis terminology

Other helpful items:
– Directory of Primary SMEs per application and/or department
– Interview tips reference sheet

We also are testers where I work, so something documenting the different testing types would be beneficial, as well as test plan and test case templates. If you use automated testing, scripts should be available for re-use.

The challenge that I have had in the past on a couple of projects and encountered again for this project is: How to gather the hidden requirements of a senior VP, who is the project sponsor, when they say that they are delegating to the project manager but do not allow the project manager to make any decisions without going back to the sponsor for approval. The sponsor will not identify any requirements up front and only looks at deliverables when it’s the hour before they are due. Then the VP changes anything that they want without regard to the end users and any gathered requirements form them. Working only from their point of view. I guess this is a question of how best to go after hidden requirements.