Microsoft Office: Microsoft Excel Spreadsheets
Microsoft Excel is the spreadsheet software that most people would use to make their inventory sheets. If you've been looking at sample inventory sheets, chances are it came from someone using Excel to create it. This is the established standard in creating inventory reports, but it comes at a price. The price is that of buying a package of Microsoft office. Which may be prohibitive if you do not already have one installed. Much of the guides in books or on the internet are of course usually written with excel in mind.
The immediate plus point of using this is that it is free! It is an open source version of the popular office workshop. Being Open Source, you do not need to pay for it and can install it on numerous computers all over the place. On the downside, it faces compatibility problems when trying to interface with other computers running without open office. Of course, for the simplest documents one can move between Open Office and Microsoft Excel easily, for complicated spreadsheet formatting that might prove difficult occasionally.
This is also another free way to make spreadsheets. But on the down side it requires a constant Internet connection. It works similar to office's excel as well as open office's spreadsheet component, but is slightly limited to the items in its Google Document toolset. It also operates in a vacuum, making for rather difficult file transfers if one is not used to it. Since it operates in your web browser, if you are using internet explorer you are liable to the junk toolbars, so its better to use firefox. It is harder to work into local file transfer as you would then have to download the file from the internet before moving it into your computer folders or thumb drive. On the other hand, since it is linked to your gmail account it makes it easy for you to send it out as an email attachment. Once again similar compatibility problems may surface with the Microsoft Office versions.
For a comparison of the two, you might want to check out this contrasting of inventory spreadsheet software.
This is the version native to the macintosh OSX, created by Apple. It actually makes for aesthetically pleasant spreadsheets and is very easy to use. It is also slightly cheaper than Microsoft Office. The downside is that it is macintosh only, and is also slightly more restricted than office. Furthermore if one is using macintosh computers, Microsoft office also happens to be available on it though it is a separate purchase.
Pen and Paper
The classical method, that is free after you obtain a pen and a paper and perhaps a ruler. The down side is of course that it is tedious. It also lacks the convenience of the modern age, and even if one scans it in it would generally exists as an image document only. To convert it to text would require a separate input on one of the software available elsewhere.
Custom Proprietary Software
This is for the very rich and successful, or very large scale companies that needs something tailored specially and specifically for their needs. If none of the above suits you, then you might consider using this.