Before you evaluate any captcha solutions you should develop a clear profile of your users. Needless to say, any captcha solution is going to make your site harder to use. If your users are older with vision difficulties deciphering a captcha can be extremely difficult if not impossible. Random characters vs real words, may impact your users. Native language may affect user's perception of what the character represents (special characters).
No recommendations here, but knowing your users and what they will tolerate and or are capable of, should have a significant impact on your decision.
I would second the Mondor's Captcha mentioned in the first answer. Unlike the reCaptcha, it is fully independent, you don't make a call to other website. Also, it allows asking for solving a simple equation (like "5 + 3") and while bots would answer "5+3", human would answer 8. Also, unlike the reCaptcha, you can customize it as much as you want and include your own alphabet (Cyrillic, Arabic, Chinese - whatever).
If you're using VB.NET as your base language (or even Classic ASP), I just found a great free CAPTCHA at TipsTrips.org. You customize the CAPTCHA you want and then download the code. The .NET download includes a ASHX handler file, two VB class files and an ASPX test page.
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我一直在几个网站上使用recaptcha，并取得了巨大成功。 我真的推荐它。
此处有一个针对 ASP.NET 的 recaptcha 控件。
我还成功使用了 this 验证码。
I've been using recaptcha for a couple of sites with great success. I really recommend it.
There is a recaptcha control for asp.net here.
I've also used this captcha with success.
在评估任何验证码解决方案之前，您应该建立清晰的用户档案。 不用说，任何验证码解决方案都会使您的网站更难使用。 如果您的用户年龄较大且有视力障碍，那么破译验证码即使不是不可能，也可能非常困难。 随机字符与真实单词可能会影响您的用户。 母语可能会影响用户对字符所代表的内容（特殊字符）的感知。
这里没有建议，但了解您的用户以及他们的容忍度和/或能力，应该会对您的决定产生重大影响。
Before you evaluate any captcha solutions you should develop a clear profile of your users. Needless to say, any captcha solution is going to make your site harder to use. If your users are older with vision difficulties deciphering a captcha can be extremely difficult if not impossible. Random characters vs real words, may impact your users. Native language may affect user's perception of what the character represents (special characters).
No recommendations here, but knowing your users and what they will tolerate and or are capable of, should have a significant impact on your decision.
BotDetect 怎么样？
免费版本也可用，但它们不会强迫您的访问者解决 2 个验证码单词。
How about BotDetect?
The free version is available as well while they don't force your visitors to solve 2 captcha words.
我赞同第一个答案中提到的 Mondor 的验证码。 与 reCaptcha 不同，它是完全独立的，您无需调用其他网站。 此外，它还允许要求求解一个简单的方程（如“5 + 3”），而机器人会回答“5+3”，而人类会回答 8。此外，与 reCaptcha 不同，您可以根据需要自定义它，并且包括你自己的字母表（西里尔字母、阿拉伯字母、中文——等等）。
I would second the Mondor's Captcha mentioned in the first answer. Unlike the reCaptcha, it is fully independent, you don't make a call to other website. Also, it allows asking for solving a simple equation (like "5 + 3") and while bots would answer "5+3", human would answer 8. Also, unlike the reCaptcha, you can customize it as much as you want and include your own alphabet (Cyrillic, Arabic, Chinese - whatever).
如果您想依赖 Google，Recaptcha 是一个很好的验证码，可以保护您的表单。
Telerik 有一个很棒的 ASP.Net 验证码控件，您应该检查一下以及。
Recaptcha is a nice captcha to protect your forms if you want to take a dependency on Google.
Telerik has a great ASP.Net captcha control available that you should check out as well.
我也在使用验证码，非常棒！
以下是 ASP.NET 插件/包装器的链接：
将 reCAPTCHA 与 ASP.NET 结合使用
I am also using the recaptcha, and it is great!
Here is the link for the ASP.NET plugin/wrapper:
Using reCAPTCHA with ASP.NET
您可以尝试 Picatcha - 使用起来非常酷，而且看起来很容易实现。 图像与文本。 相当可定制
You could try Picatcha - pretty cool to use and looks easy to implement. Images vs text. Fairly customisable
如果您使用 VB.NET 作为基础语言（甚至经典 ASP），我刚刚在 TipsTrips 找到了一个很棒的免费验证码.org。 您可以自定义所需的验证码，然后下载代码。 .NET 下载包括一个 ASHX 处理程序文件、两个 VB 类文件和一个 ASPX 测试页。
If you're using VB.NET as your base language (or even Classic ASP), I just found a great free CAPTCHA at TipsTrips.org. You customize the CAPTCHA you want and then download the code. The .NET download includes a ASHX handler file, two VB class files and an ASPX test page.