Create a custom report type (Setup -> Create -> Report Types).
Call it "Accounts With Timesheets" or something like that, starting object would be Account.
Relationship - I think it'd be best to set it to "with or without".
On the next page you'll be selecting fields to include (scroll down to the "Edit layout"). On the right side there will be blue "view" box with option to add fields related via lookup. You can use this to go from "Account Type 1" to it's parent "Account Type 2".
Add as many fields as you need, maybe also create a new section, call it "Parent Account" and move the new fields there (will be easier for users to identify which field they want to use in the report).
Notes:
- The problem with custom report types is that they don't update automagically whenever you add a new field - standard report types are better. You'll have to remember to add new Account / Timesheet fields here whenever user comes to you couple weeks later that he can't report on something he'd like to see.
- Report types can span over 3 objects (4 if there's Master-Detail involved). If there are some complex things under the Timesheet object you might decide to create a report type that starts from timesheet, goes down to these objects and you can always "go up" with this "add fields via lookup".
- There's no easy way to transform report created with one report type to use another one. Plan ahead or you'll have to recreate them from scratch.
It's a few months later and I finally found the solution to this problem -- and it turns out it's already built into Salesforce (although I believe it's only available in the enterprise edition and above): Partner Portals. You don't actually have to use the core partner portal functionality, but by activating the Partner object, you automatically get a many-to-many relationship between accounts (which is exactly what I wanted). Here's what I did:
Turn on the Partner object: Setup > Customize > Partners > Settings; then check the box to enable partners and hit save.
Add the Partner Account field to the Account Layout: Setup > Customize > Accounts > Page Layouts; edit the default page layout and just drag the Partner Account field somewhere. It’s actually not editable, but I guess it’s nice to see…or maybe making it visible triggers its functionality (not sure).
To make an account a partner account, you must first create it and save it, and then you'll see a button that says “Manage External Account”; click on it an select “Enable As Partner” – now the Partner Account checkbox will be checked.
Then, the way you link an account with a partner account is you scroll down to the bottom of the account page to Partners related list. Click on “New” and select a partner account, give it a role, and save.
If you want to then run a report showing all contacts for a specific account's partners, you'll need to create a custom report with the primary object being accounts, the secondary object being partners, and the tertiary object being contacts. When you run the report, filter it by the original account, and you'll see all contacts for that account's partners!
Best Answer
This IS shown in the Data Usage page ;)
On every User's page you have something like "Used Data Space 414 KB [View]". It leads to page:
Also - "Setup -> Storage Usage" will have a section on the bottom with "top offenders" in data & file space.