I use GNUCash. It's a bit more like Quickbooks than plain Quicken, but it's not all that complicated. Probably the most difficult part is understanding the idea of income accounts.
- freely available on Windows, OSX and Linux
- open source
- online transaction pull from supported banks / other institutions, with bayesian transaction matching to eliminate duplicates
- supports multiple currencies, and most any ticker symbol you can imagine
- scheduled transactions can help you plan expenses and income before they occur
- the file format is either xml, sqlite or SQL database (mysql, postgres), so the programmatically inclined can
- a budget sheet and historical estimator (But really i just prefer scheduled transactions and a spreadsheet)
For short term planning, I use scheduled transactions. If I'm spending more than I have, it'll show up here. Every paycheck and dollar spent or invested is recorded with the exact date I anticipate it will happen, 30 days in advance. If that would overdraw my checking account, the Future Minimum Balance field will go negative and red. This lets me move float to higher interest savings and retirement funds, and avoids overdraft fees or other mishaps. By looking 30 days ahead in detail, I have enough time to transfer from illiquid assets.
For longer term planning, I keep a spreadsheet around that plans out annual expenses. If I'm spending more than I earn, it shows up here. I estimate everything: expenses, savings, taxes, and income. I need this because I have a lot of expenses that are far less frequent than monthly or paycheck-ly. The beauty of it is that once I've got it in place, I can duplicate the sheet and consider tweaks for say taking a new job or moving, or even just changing an insurance plan (probably less relevant for those with access to NHS). Especially when moving to take a new job, it's not as straightforward as comparing salaries, and thus having a document for the status quo to start from lets you focus on the parts that changed.