Excel templates can be great time savers. But perhaps more importantly, they are educational. You can learn a great deal from seeing how someone else has put together a spreadsheet.
Vertex42 is one of the largest providers of original spreadsheet templates in the world. Below are some other sites that either provide repositories of templates (uploaded by the author), compilations of templates (files shared hopefully with author's permission), or original providers (created by the authors of the site).
Note: Vertex42 is opposed to the practice of hosting files created by other authors without the authors' specific permission.
Microsoft is an obvious place to look for templates. They list many templates on their template site, but you can find more templates by opening Excel and going to File > New and doing a search.
This site founded Janet Dagostino creates some of their own templates and also offers a way for template designers to sell their templates.
A site by Onur Yilmaz providing Excel solutions, services, and templates for personal and business use.
This site by Dinesh Mohan provides many free templates with a lot of interesting functionality.
This site by Erin Landry-Jones provides a variety of templates, including spreadsheets that are especially useful for people who sell products on Etsy.
[Repository] This site has a great collection of professional and powerful Excel templates (including dashboards and calculators). It looks like most of them are free to download. People can upload their own templates, and this site lists the Author.
[Compilation] Large list of free excel templates related to financial management, compiled by Matt H. Evans, CPA, CMA, CFM.
Browse a large collection of Excel Templates. Most with money back gaurantee, some with free software.
Purpose-built Excel templates for financial analysis, management and business decision making.
JaxWorks™ has many free accounting and financial templates, including forecasting, budgets, cashflow, sales, and other financial analysis spreadsheets.
Loan and mortgage spreadsheets and calculators, vehicle buy vs. lease, buy home vs. rent, and more.
Set of Microsoft Excel workbooks for data analysis, to replace and enhance the tools provided in the Excel Data Analysis Toolbox addin. The workbooks are designed to be step-by-step guides. (Free for personal use)
Philip Taylor provides a couple useful financial spreadsheets on his blog.
This site offers a collection of spreadsheets for genealogy, including federal and state census forms.