Perfect alternative to legacy commercial applications but ...

Open Source is good for any organization. There are plenty of reasons to go for this kind of solutions. But before selecting a solution, you need to learn a few more things.

The quality of Open Source projects
The quality of the Open Source projects is a direct consequence of what they are : a wide open-minded community of developers and testers. This bring to you a software with a high quality level. But don't dream, a project with few users and few developers for it won't have the quality of an Apache, Linux or OpenOffice. This is why the selections criteria include the need of an estimation of the project community.
So don't go for an Open Source solution because in general they all are of better quality. Still do a review on a per-project basis.

About technical features

When choosing a solution, keep considering both Open Source and commercial proprietary. Maybe the features you need are not present in any Open Source Solutions. This does not mean that Open Source solutions lack some interesting features regarding proprietary solutions. No, but when you compare different solutions - even only proprietary ones - there will be features present in one and not in the other and vice-versa. There is not two different software with the same features, it's true in proprietary software, it is true in Open Source also. Thus don't eliminate Open Source or proprietary software just because you think they may lack some features.
Therefore, before selecting among Open Source projects, you will first compare solutions regarding their features and not if they are Open Source or not.

The costs
Don't choose an Open Source Solutions just because it is Open Source. Especially if all you want is to cut the costs. Sometimes, the costs are not so obviously cut down. When an organization decides to go for Open Source solution(s), even if they are cheap, the result may not be as cheap as one would have thought.
For instance, if you decide to switch from Windows servers to Linux servers due to the cost of licenses, don't forget that you will need to re-train your system engineers and administrators, that you may have to switch also some applications or that maybe you will have to hire new people. As a result, your Windows-to-Linux migration will have a low acquisition cost but implementation and administration may cost you more.
So if reducing the costs is your only goal with Open Source, please be careful on how you will do this. The ROI and / or TCO of a MySQL installation on Windows can be better lower than one on Linux if you have only Windows system administrators in your organization.

Extending the solution
Technical features of a software to answer a need or problem is what you are looking for. So If you miss something with your current solution, you need to look at another one. But with Open Source you know that there is a possibility for you to add the missing feature(s). And this is one of the strongest advantage of an Open Source software, if something you want isn't in there, you can have it added quicker than with proprietary software.
But if you want something more into the Open Source application you plan to use, attention :
  • Don't underestimate the cost it may add if you do the development yourself or with the help of a service company.
  • When you modify something yourself, if this change is not reported in the original software, for each update / upgrade you will need, you will add a supplemental layer of difficulties.
  • If you don't want - can do the development yourself, it can be sometimes hard to convince the developers community of this project to insert the change you are interested in. Even if they are open-minded and quicker to respond positively than with proprietary software.
You can avoid these problems if you choose an application supporting extension through a plugin architecture. Doing extensions as plugins won't impact the core of the application and won't be a hassle when doing updates. But this will require development skills of course and may rise the total cost of your "cheap" solution.
In some area, like in CMS, there so many different Open Source solutions that it seems impossible to not find the one with all that you need out of the box. So, once more, the selection phase is important.

Don't be afraid by what has been said here. If you select carefully your Open Source solution without under-estimating  the risks linked to the implementation and migration, you will have a cheaper alternative to any proprietary solution.