Given the collapse of the stock markets, now is the time to buy (buy low, sell high, remember?).
However, it is also important to keep in mind basic investment advise, such as the need to diversify risks (between types of assets, sectors, geographically, etc.).
1) Don't invest any cash you will need in the next 3-4 years (keep that money in a high-yield savings acct. or Certificate of Deposit, in an institution guaranteed by the FDIC; for example, INGDirect.com). Certainly don't borrow money to invest!
2) Of the difference, invest one third in broad sector or geographic stock indices (not individual companies). You can do so through Exchange-Traded Funds (ETFs) at sharebuilder.com
3) Another third you can invest in broad bond indices (I particularly like TIPS, Treasury Inflation Protected Securities, which are protected for inflation, as the name suggests). Also available as ETFs.
4) The rest you can invest in commodity-linked ETFs (such as gold, energy, minerals, etc.), sustainable energy and sustainable development ETFs, or in "social lending" sites such as prosper.com and upstart.com (but lend only to low-risk borrowers, classified AA, A or at minimum B).
5) If you have kids that will go to college, start a College Savings Plan account (preferably a pre-paid tuition program like Virginia's VPEP; or a 529 Plan).
6) If you are elegible, start a Roth Individual Retirement Account (IRA) (you can also do so through sharebuilder.com), which has tax advantages (direct contributions can be withdrawn tax free at any time).